Emission shader that glows on the center of the object and is dark on the edges
The disconnected Shader socket on the Mix node will be providing what is effectively a ‘black’ material. The Mix mixes between what is on the’top’ input with what is on the ‘bottom’ socket based on the Factor - with 0.0 and less giving the ‘top’ one on the output, 1.0 and above givinn the ‘bottom’ one on the output and values between 0.0 and 1.0 giving a mix between them. Therefore, to ‘invert’ your result you could either manipulate the Factor (eg, mathematically subtract it from one) or, alternatively, just swap te Shader inputs - so the emission is connected to the bottom one and the top one is unconnected.

Another option here is to add a Color Ramp between te Fresnel and Factor and you can then manipulate the ramp to change the effect.





EDIT : For a more controllable transition you could use vector maths and a Color Ramp as follows :